The present invention relates to SRNS Relocation in a UMTS network and more particularly to the transfer of radio resources between a SRNS and a target RNS during such Relocation.
FIG. 1 illustrates schematically a part of a Universal Mobile Telecommunications System (UMTS) network. The network includes a core network part 1, which may be a network handling voice calls using UMTS Mobile-services Switching Centres (UMSCs) or may be a data network such as a General Packet Radio Service (GPRS) network including Serving GPRS Support Nodes (SGSNs). In FIG. 1 the UMSCs and SGSNs are indicated generally with the reference numeral 2. A subscriber or User Equipment (UE) 3 is coupled to the core network 1 via an access network 4 referred to as a Universal Terrestrial Radio Access Network (UTRAN). More particularly, the UMSCs/SGSNs 2 are connected to Radio Network Controllers (RNCs) 5,6 of the UTRAN 4 over an interface referred to as the Iu interface.
Each RNC 5 forms part of a Radio Network Subsystem (RNSs) 7,8 which also comprises a set of Base Transceiver Stations 9 referred to in UMTS terminology as Node B""s. The interface between a RNC 5,6 and a Node B 9 is known as the Iub interface. A node B 9 provides the connection point for a UE 3 to the UTRAN 4, and the interface between the Node B 9 and the UE 3 is known as the Uu interface. The RNS (RNS 7 in FIG. 1) which connects a UE 3 to the core network 1 at any given time is referred to as the Serving RNS (SRNS) for that particular UE 3.
FIG. 2 illustrates in very general terms the bearer structure used by UTRAN to carry user data between the UE 3 and the core network 1. When it is required to establish a user plane connection, the responsible UMSC or SGSN 2 instructs the UTRAN 4 to establish a logical connection between the UMSC or SGSN 2 and the UE 3. This logical connection is referred to as a Radio Access Bearer (RAB). The established RAB inherits requirements of the requested UMTS service, e.g. Quality of Service, etc. Based on the inherited requirements of the RAB, the RNC 5,6 establishes user plane connections with the core network 1 (i.e. UMSC or SGSN 2) and with the UE 3. The connection between the RNC 5,6 and the core network 1 is referred to as the Iu bearer whilst the connection between the RNC 5,6 and the UE 3 is referred to as the Radio Bearer (RB). Both of these bearers represent further logical channels, with the RNC performing a mapping between them. The bearers themselves are mapped onto appropriate traffic channels for transmission over the respective interfaces (Iu and Uu).
A single UE 3 may be associated with one or more RABs. For example, a UE 3 may simultaneously make use of one RAB established for a voice call, and another RAB established for a data call. The RNC 5,6 uses RAB identifiers (allocated by the core network) to distinguish between these different RABs. In the same way, a RB identity is used to distinguish between RBs. During the setting-up of a new user plane connection, a RAB and a RB are implicitly associated with one another by the RNC.
An important feature of mobile telecommunication systems is the ability to allow subscribers to move within (and outside) particular networks. This often requires a process known as SRNS relocation, in which an existing SRNS is replaced by a target RNS. Considering a scenario known as Hard Handover where a UE moves out of the area of responsibility of a first RNS, into a new RNS, it is necessary that the RNC (i.e. the Serving RNC (RNCs)) of the SRNS be replaced by the RNC (referred to during the transition phase as the Target RNC (RNCT)) of the second RNS, as the access RNC for the UE. In some circumstances, the Target RNC may immediately become the Serving RNC, with the user plane connection being established between the Target RNC and the core network. In other circumstances, the user plane connection extends between the Target RNC (which is thereafter referred to as a xe2x80x9cdriftxe2x80x9d RNC) and the core network via the Serving RNC, where the interface between the drift RNC and the Serving RNC is known as the Iur interface. The network may subsequently decide to convert the drift RNC into the Serving RNC, establishing a direct user plane connection to the core network.
In either scenario, the SRNS relocation process may be initiated by the Serving RNC sending to the core network (UMSC or SGSN) a Relocation Required message. This message is currently defined in the Radio Access Network Application Part (RANAP) protocol (UMTS TS 25.413) and carried over the Iu interface. The core network responds to receipt of a Relocation Required message by sending a Relocation Request message to the Target RNC. The Relocation Request message is also defined in the RANAP protocol.
The Relocation Request message contains the identities of the RABs to be transferred. Both the Relocation Request message and the Relocation Required message contain an RRC Initialisation Information container. This container is defined in the Radio Resource Control (RRC) protocol (UMTS TS 25.331), and contains amongst other things the identities of the RBs to be transferred as well as details of the mappings between the RBs and the lower layer attributes, i.e. to logical, transport, and physical channels. It is noted that the RB identities and the mappings between the RBs and the lower layer attributes are contained in a container which is passed transparently by the core network.
The inventors of the present invention have recognised that in order to deal with a SRNS Relocation situation for a given UE, where that UE has multiple RABs allocated to it, the Target RNC must have a knowledge of the mappings between the RABs and the RBs. Under the current proposal, neither the Relocation Request message nor the Relocation Required message contain sufficient information to perform this mapping. The result is that confusion might arise at the Target RNC, with the Target RNC associating RBs with the wrong RABs. In such a situation, calls may be incorrectly connected and resources allocated inappropriately.
According to a first aspect of the present invention there is provided a method of performing a SRNS Relocation in a UMTS network for a given UE, the method comprising sending a Relocation Required message from the Serving RNC to the core network and sending a Relocation Request message from the core network to the Target
RNC, the Relocation Required message containing a RRC Initialisation Information container which is incorporated transparently by the core network into the Relocation Request message, and the RRC Initialisation Information container in turn containing information enabling a mapping to be made between RABs and RBs for the UE.
Preferably, where an RRC Initialisation Information container contains a Radio Bearer Information element, said mapping comprises for each RB, a RAB identity to which that RB corresponds, the RABs and the RBs being identified by RAB and RB identifiers respectively.
The mapping information contained in the RRC Initialisation Information container may comprise NAS binding information.
According to a second aspect of the present invention there is provided UMTS telecommunications system comprising at least one core network and a UTRAN, the UTRAN comprising a plurality of RNCs, pairs of which are arranged to communicate with each other, via said core network, in the event of SRNS Relocation, each RNC being arranged to generate and send a Relocation Required message to the core network in the event that the RNC is the pre-existing SRNS for a UE, the Relocation Required message containing a RRC Initialisation Information container which in turn contains a mapping information enabling a mapping to be made between RABs and RBs for the UE, and each RNC being arranged to receive a Relocation Request message from the core network in the event that the RNC is the Target RNC, the Relocation Request message containing said RRC Initialisation Information container.
It will be appreciated that the core network is arranged to translate a received Relocation Required message into a Relocation Request message for transmission to the Target RNC. The RRC Initialisation Information container is copied transparently in this process.